; 4-4
; 该版本不区分基数长度与偶数长度
(defun mirror? (s)
	(let ((len (length s)))
		(do ((forward 0 (+ forward 1))
				 (back (- len 1) (- back 1)))
			((or (> forward back)
					 (not (eql (elt s forward)
										 (elt s back))))
			 (or (> forward back))))))

(position #\a "fantasia")
(position #\a "fantasia" :start 3 :end 5)
(position #\a "fantasia" :from-end t)
